code: def merge_patterns(flavour, pattern_a, prefix_a, pattern_b, prefix_b):
      """
      Merge two path specification patterns.
  
      This implementation merge two path specification patterns on different
      bases. It returns the longest common parent directory, and the patterns
      based on this new base directory.
      """
      if not pattern_a:
          return pattern_b, prefix_b
      if not pattern_b:
          return pattern_a, prefix_a
  
      longest_common_dir = flavour.commonpath([prefix_a, prefix_b])
      new_pattern_a = _change_dirname(
          prefix_a, pattern_a, longest_common_dir
      )
      new_pattern_b = _change_dirname(
          prefix_b, pattern_b, longest_common_dir
      )
  
      if len(prefix_a) <= len(prefix_b):
          merged_pattern = new_pattern_a + new_pattern_b
      else:
          merged_pattern = new_pattern_b + new_pattern_a
  
      return merged_pattern, longest_common_dir
  
file_path: dvc/dvc/pathspec_math.py
imports: from collections import namedtuple
  from pathspec.util import normalize_file
  from dvc.utils import relpath
label_desc: Use image node_id 7 for calling a global function with example usage: merge_patterns(flavour, pattern_a, prefix_a, pattern_b, prefix_b) and returns: merged_pattern,  longest_common_dir, pattern_b,  prefix_b, pattern_a,  prefix_a